The present invention relates to an electrode adapted for use in an electrolyte circulation-type cell stack secondary battery.
Generally, a metal-halogen cell, e.g., zinc-bromine cell, one of the cells with which the electrode of this invention can be used, is basically constructed as shown in FIG. 1. The cell comprises a unit cell 1 divided by a membrane (separator) 2 into a positive electrode chamber 3 and a negative-electrode chamber 4, a positive electrode 5 positioned in the positive electrode chamber 3 and a negative electrode 6 positioned in the negative electrode chamber 4. A positive electrolyte is circulated through the positive electrode chamber 3 from a positive electrolyte storage tank 7 by a pump 9 and a negative electrolyte is circulated through the negative electrode chamber 4 from a negative electrolyte storage tank 8 by a pump 10. Numerals 11 and 12 designate valves adapted to open during the charging and discharging, respectively, to circulate the electrolyte.
Practically, a bipolar type of cell stack battery comprising a stack of unit cells of the type shown in FIG. 1 has been in use. FIG. 2 is an exploded perspective view showing an example of the bipolar type of cell stack battery. As will be seen from FIG. 2, the bipolar type of cell stack battery is constructed by alternately stacking a plurality of separating means 17 each having a separator and a plurality of electrode means 18 each having a positive and negative electrode, arranging terminal boards 13a and 13b on both sides of the stack and inserting a bolt 14 through each of bolt holes 19 formed in the respective means thereby holding all the means together as an integral assembly. The terminal board 13a is formed with a positive electrolyte inlet 15 and a negative electrolyte inlet 16 and the other terminal board 13b is formed with a positive electrolyte outlet 15a and a negative electrolyte outlet 16a. Now referring only to the flowing path of the positive electrolyte, the positive electrolyte supplied from the positive electrolyte inlet 15 passing through a manifold 32 formed in each electrode means 18, is introduced into a microchannel 36 of its electrode member 30 through a channel 34, rectified and then supplied to the positive electrode surface of the electrode member 30. Then, the positive electrolyte flows to the positive electrolyte outlet 15a through a microchannel 36a, a channel 34a and a manifold 32a on the outlet side of each electrode means 18 and a manifold 17a of each separating means 17 and is returned to the electrolyte storage tank. The negative electrolyte flows out from the negative electrolyte outlet 16a through another flowing path similarly from the negative electrolyte inlet 16 through the negative electrode side of the respective electrode members 30.
In such circulation of the electrolyte, the flowing velocity becomes extremely nonuniform over the surface of the electrode member 30 with the resulting eddy and stagnation in the section where the electrolyte flows onto the electrode member 30 from the microchannel 36. In the case of the metal-halogen cell, such eddy and stagnation makes the concentration of the electrolyte nonuniform so that an electrochemical reaction on the negative electrode side becomes nonuniform and numerous dendrites are formed on the negative electrode. Such formation of dendrites has the disadvantage of making the current density nonuniform and deteriorating the efficiency of the cell.